The present invention relates to a process for the improved separation of substances which hinder the recovery of the fissionable materials uranium and plutonium, and for the improved separation of the fissionable materials to be recovered in a reprocessing process for spent, irradiated nuclear fuel- and/or fertile materials. In the recovery process, irradiated nuclear fuel- and/or fertile materials are dissolved in strong, aqueous nitric acid to form an aqueous phase or starting feed solution containing the fissionable materials and the hindering substances, the fissionable materials are then transferred from the aqueous phase into an organic phase by an organic extraction agent solution, and thereafter the fissionable materials are separated from each other in purified form and are stripped into aqueous solutions.
The so-called Plutonium-Uranium-Reduction-Extraction Process (PUREX Process) has gained wide acceptance for the reprocessing of irradiated nuclear fuel- and/or fertile materials. This process comprises dissolving the nuclear- and/or fertile materials in strong nitric acid as described above, which selectively affects the fuel only, and then of a first extraction cycle, in which uranium and plutonium together are extracted from the aqueous nitric acid into an organic tributylphosphate(TBP)/kerosene solution and thereby are separated from the main part of the harmful substances which remain in the aqueous nitric acid. Subsequently, plutonium is separated from uranium by selectively reducing the plutonium and stripping it into an aqueous phase. The uranium remaining in the organic phase is then stripped into an aqueous phase. This ends the first extraction or purification cycle. (As used herein, an extraction or purification cycle or a cycle refers to an extraction operation into an organic extractant, one or more scrubbing or washing operations, and a stripping operation into aqueous solution.) Then, two further purification cycles each for uranium and plutonium are conducted after which uranium and plutonium are present in aqueous solutions. Until now, this process has been required in order to achieve the usual product specifications for uranium and plutonium that must be met at this time. The product specifications refer to the product purities in reference to certain fission product nuclides or other foreign nuclides.
In the following table, the required decontamination factors are presented correlated with the cooling time of the irradiated nuclear fuel material after a burn-up of 33,000 MWd/t. Decontamination factors (DF) can on the one hand obtained from the concentrations present in the solution, and they are given by the ratio of concentration before a purification step to concentration after it, or they can represent the decline in activity from before purification to after purification. In columns 3 and 4 of the table, the respective total decontamination factors (DF.sub.tot) for uranium or plutonium are given, that is, the factors by which the concentrations of impurities in uranium or plutonium, respectively, must be reduced in order to obtain the required impurities of the uranium product or of the plutonium product, respectively, after completing the reprocessing.
TABLE __________________________________________________________________________ Required Decontamination Factors in Relation to the Cooling Period of the Burnout: 33000 MWd/t Uranium Puri- Cooling fication DF for Plutonium-Purifi- Period Activity.sup.1 DF.sub.tot Relevant FP-DF Np.sup.5 (as cation DF for [Years] [Ci/mol U] in U.sup.2 in Pu.sup.3 in U.sup.2 Pu.sup.4 (as Pu conc.) .alpha.-activity) U.sup.3 (as U Np.sup.3 (as Np conc.) __________________________________________________________________________ 0.5 475 4 .multidot. 10.sup.6 5 .multidot. 10.sup.6 Zr/Nb--Ru 9 .multidot. 10.sup.5 70 2700 28 2.6 .multidot. 10.sup.6 3.4 .multidot. 10.sup.6 1.0 235 2 .multidot. 10.sup.6 2.5 .multidot. 10.sup.6 Zr/Nb--Ru 9 .multidot. 10.sup.5 70 2700 28 7.5 .multidot. 10.sup.5 9.8 .multidot. 10.sup.5 3.0 58 4.9 .multidot. 10.sup.5 6.1 .multidot. 10.sup.5 Ru 9 .multidot. 10.sup.5 70 2700 28 1.3 .multidot. 10.sup.5 1.7 .multidot. 10.sup.5 7.0 24 2.3 .multidot. 10.sup.5 2.5 .multidot. 10.sup.5 Ru 9 .multidot. 10.sup.5 70 2700 28 9.7 .multidot. 10.sup.3 1.1 .multidot. 10.sup.4 __________________________________________________________________________ .sup.1 Sum of the activities of 95Zr/Nb, 106 Ru, 137 Cs, 144 Ce .sup.2 Specification of the Cogema Firm, France .sup.3 Specification of the ALKEM Firm, Germany .sup.4 WAK Specification .sup.5 .alpha.: 15000 dpm/gU, from this Pu: 5000, Np: 10000
It is known that the different harmful substances, among which are the fission products, are removed with different rates of efficiencies under the same process conditions. The fission products zirconium/niobium and ruthenium are comparatively difficult to remove. If it is assumed that the other harmful substances are easier to remove, then it can be concluded that the fission product decontamination of uranium or of plutonium, respectively, required during the recovery process is determined almost solely by the nuclides Zr-95 and Ru-106 as well as their daughter products. The DF required for these are presented in the table in column 5 as "Relevant FP-DF". After cooling times of three years and more, the main problem of fission product separation is practically to be regarded only as the effective separation of Ru, because Zr is decomposed and all other nuclides are separated without problem under the extraction conditions of the process. The lower values for Relevant FP-DF compared to the values of DF.sub.tot occur because part of the residual fission product nuclides in the uranium- or plutonium-product, respectively, are more harmful and therefore require a higher specific DF. This, however, has no further effect, because they can be more easily separated. Columns 6 and 7 contain the decontamination factors for plutonium and neptunium by which the uranium must be purified. Columns 8 and 9 show the corresponding DF for uranium and neptunium for plutonium purification. It can be concluded from columns 6 to 9 that the DF for Pu and Np in uranium or for U and Np in plutonium, respectively, are not a function of the cooling time. In contrast, the required DF for the selected fission products show a significant dependence on the cooling time. While already in the first cycle of the PUREX process, decontamination factors for ruthenium are achieved which only are approximately one order of magnitude below the DF required for three years cooling time, in the known installations for reprocessing irradiated nuclear fuel materials, in the product uranium usually the required DF for Pu and Np is reached only after a total of three cycles.